The present invention relates to a transmitting-receiving apparatus for use with a TDMA (time-division-multiple-access) system transmitter-receiver in which a transmission timing and a reception timing are different from each other.
In radio transmitter-receivers like a portable radiotelephone, a transmission signal, processed for transmission, is amplified by a transmission amplifier and supplied to a transmission antenna from which it is transmitted via radio waves. In order to enhance the receiving state of the transmission signal transmitted via radio waves on the reception side, the transmission signal should preferably be transmitted with a transmission power raised as high as possible by the transmission amplifier.
Further, in order to accurately demodulate the transmission signal on the reception side, it is important to prevent a distortion from being generated in the transmission signal when the transmission signal is amplified by the transmission amplifier as much as possible. In particular, when a phase-modulated signal is transmitted as a transmission signal, there is then the very large possibility that a distortion generated in the transmission signal will cause a bit error on the reception side. Therefore, it is very important to prevent the distortion from being generated in the transmission signal.
An amplifier called a class-A amplifier where transistors are operating permanently on linear portions of their transfer characteristics should preferably be used as the amplifier with minimum distortion.
Battery radio transmitter-receivers like a portable radiotelephone use a limited electric power for transmission processing. Therefore, from a standpoint of battery duration, the electric power used by the amplifier should be limited as much as possible.
Accordingly, use of the class-A amplifier is not preferable from a power consumption standpoint. Specifically, although the class-A amplifier has less distortion, it should be constantly energized even in the absence of a signal. Moreover, the class-A amplifier has a poor efficiency so that, if the transmission output level is raised, then the power consumption thereof is increased considerably.
An amplifier with high efficiency, e.g., the class-C amplifier should preferably be used in order to suppress a power consumption. The class-C amplifier is of the power-amplifier stage where bias of the amplifying element is adjusted so that the drive signal voltage produces output current for less than half of the total cycle. Thus, the class-C amplifier can amplify an input signal with a small electric power and can prove a satisfactory amplifying efficiency.
However, although the amplifying efficiency of the class-C amplifier is high, a distortion is generated in an amplified signal. Therefore, when the class-C amplifier is used as a transmission amplifier, it is necessary to provide a circuit for eliminating the distortion generated in the amplified signal. As the circuit for eliminating the distortion generated in the amplified signal, there is known the following circuit. More specifically, an output from the amplifier is detected by a detecting circuit and an error signal (i.e., distortion component) is detected from a detected signal and a detected distortion component is fed back to the amplified signal, thereby the distortion can be eliminated. If such distortion eliminating circuit is provided, then the transmission processing circuit becomes complicated.